Farewell to Ironwood Forests: The end of an ecosystem in central Sumatra, Indonesia

The number of Ironwood trees in Sumatra has greatly reduced because of heavy demand for the timber.

Just over 25 years ago, I was lucky enough to be working at the University of North Sumatra and writing what became the first in the Ecology of Indonesia series. During that time I did quite a bit of travelling around Sumatra, and it was exciting to find what was thought to be the last bit of pure ironwood forest near Rimbo Kulim not far from Muara Tembesi in Jambi province, a region I’ve been driving around again this last few days.

Ironwood forest in Sumatra is of special interest because of its extremely low diversity of tree species, being dominated (unsurprisingly) by the ironwood, which glories in the scientific name Eusideroxylon zwageri.

Ironwood, a laurel, is found not just in southern Sumatra, but also in Borneo and in the southern Philippines. It grows to 50 meters tall and 2.20 meters in diameter, with a lovely warm red-brown bark, large leaves and heavy fruits. Its timber is economically very valuable because of its strength and durability; it can resist rotting for up to 40 years even when in constant contact with wet soil, or for a century in drier conditions.

Its range in Sumatra has been greatly reduced by the heavy demand for its timber. It has been shown that ironwood could be quite easily managed for sustainable production because, when felled, numerous coppice shoots grow from its base, but this resource has been squandered. At the same time, however, it is the official tree of Jambi, and government buildings in the capital have to be roofed with ironwood shingles. It is also the tree planted by VIPs on special occasions – as when Britain’s Prince Charles visited the Harapan Forest Initiative.

Ironwood forests varied in composition from area to area – from pure stand to forests where it is but one of many species but clearly the dominant one. Even back in 1983 it was hard for us to find an unexploited area. We eventually found the Rimbo Kulim patch, though the sound of chainsaws was all around us. Some 95% of the trees were ironwood.

Seeds of an ironwood tree.

There is of course a general and popular view that in tropical regions where climate and soils are favorable for plant growth, high species diversity in natural ecosystems is necessary for ecological stability. Dan Janzen has suggested that species-poor stands of trees can occur if they produce large and toxic seeds on a more or less continuous basis. Some mangrove species do this and so, it seems, does ironwood. I had some analyses done on the seeds and it seemed they were rich in lectins – a sort of natural glue which would gum up the mouths of predators such as porcupines and act as digestion inhibitors.

But this whole area of Jambi is now converted from forest to plantations of oil palm and rubber. It’s a funny feeling knowing that I was not just one of the few people to see this unusual ecosystem but among the last. An ecosystem removed from the planet. The bits and pieces of analysis I did are of no more than historical interest now. But, just for the historical record, I wish I’d taken more photos.